1. Field of the Invention
The present Invention relates to a lightweight and low thermal expansion composite material. The composite material of the present invention can be utilized for an automobile, an aircraft and the like, and it is especially suitable for making a pump rotor of a supercharger for an automobile.
2. Description of the Prior Art
A lightweight and low thermal expansion composite material as disclosed in Japanese Examined Patent Publication (KOUKC)KU) No. 19411/1976 has been proposed. The composite material includes a homogeneously dispersed mixture including "shirasu" balloons, aluminum, zinc and tin, or an alloy made mainly of these metals. The "shirasu" balloons have a particle size falling within a range of 30-600 micrometers.
In general, this lightweight and low thermal expansion composite material is produced as follows. First, a "shirasu" balloon aggregate is formed by closely filling up a mold with "shirasu" balloons which have a particle size falling within an appropriate range. After forming the "shirasu" balloon aggregate, the mold is depressurized and a molten metal is introduced and forced into the interstices between the "shirasu" balloons of the aggregate by atmospheric pressure.
The above-mentioned "shirasu" balloons are produced from volcanic ash. The "shirasu" balloons are minute hollow particles and generally have irregular shapes. Because the "shirasu" balloons are not uniform in shape it is hard to fill up a mold closely or densely and evenly or uniformly with the "shirasu" balloons. Therefore, there is a limitation to increase a volume content of the "shirasu" balloons in the conventional lightweight and low thermal expansion composite material. Accordingly, there are limitations to a further lightweight the conventional decrease in weight of and low thermal expansion composite material made of the "shirasu" balloons, and to further making the material less likely to expand. Additionally, there is another problem that it takes a lot of time to fill up a mold with the "shirasu" balloons, thereby resulting in decreased production efficiency.
Further, the particle size of the "shirasu" balloons influences greatly the physical properties of the conventional lightweight and low thermal expansion composite material. When the particle size of the "shirasu" balloons is too large, the strength of the composite material decreases significantly. In addition, the "shirasu" balloons having a large particle size usually have an uneven shell thickness; and therefore, they are more likely to fracture due to stress concentration during molding. Accordingly, the light alloy particles are not disposed uniformly in the interstices between the "shirasu" balloons, thereby making the physical properties of the conventional composite material less uniform. On the other hand, when the particle size of the "shirasu" balloons is too small, molten metal flowability decreases thereby resulting in a prolonged molding time.